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380 CHEMICAL ENGINEERING
A. properly purified and well-filtered solution will not give much trouble on account of foaming, but all sugar juices contain considerable amounts of lime and silica, which will form a hard scale on the tubes. This scale is usually removed by boiling with a weak soda ash and acid solution. The Standard type has the advantage over all other constructions as it can be cleaned mechanically and is therefore used to a large extent in the cane-sugar industry. Evaporator shells are usually made of cast iron and sometimes of steel. Tubes are of brass or copper.
Glucose.—One bushel of corn will give about 14 gal. of liquor, and the first concentration is done in a triple-effect evaporator from 15 to 30°Be*. with from 5 to 10 Ib. of exhaust steam pressure and a vacuum of from 27 to 28 in. The capacity is from 2H to 3 gal. per square foot. Usually the standard and the horizontal-type evaporators are used; also the semi-film. The latter has the advantage that it will prevent foaming and facilitate cleaning. The second concentration from 30 to from 44 to 45°Be. is done in a single-effect of the standard or horizontal type. The capacity is from M to .1 gal. per square foot, and the horizontal type must be of special construction so as to leave sufficient space between the tubes for the discharge of the heavy syrup. On account of the calcium sulphate in the weak liquor, the tubes in the multiple effect will foul very rapidly, and have to be cleaned either chemically or mechanically. The liquors are apt to foam, and special precautions in the form of extra vapor space and separators are to be taken. Evaporators are usually built with cast-iron shells and copper tubes.
Steepwater.—One bushel of corn will give from 4 to 5 gal. of steepwater and this is concentrated in vertical or horizontal-type evaporators from 2K to 23°Be*., and sometimes 300Be\ Usually single effects only are used, as the higher temperatures of multiple effect would give the liquor a dark color. A concentration over 23°B6. will also darken the liquor. The capacity is from IK to 2 gal. per square foot, but tubes foul quickly on account of a coating of gluten, which has to be dissolved by strong alkaline solutions. The liquor is always slightly acid, and only cast-iron shells with copper tubes can be used. High-vapor space and special entrainment separators are necessary to avoid losses by foaming.
Maltose.—Malt extract or baker's extract with a high percentage of diastase are made from pure malt, and one ton of malt will give about 4 tons of 14°Balling, and 6 tons of 10°Balling weak extract. This liquor is usually concentrated to 50°Balling in a single or double effect of the horizontal or semi-film type, with a capacity of from 2H to 3K gal. per square foot, with atmospheric pressure or less in the steam chest and 28 in. vacuum. For all diastatic extracts, the boiling temperature must not exceed 60°C. The 50°Balling extract is concentrated to from 75 to 80°. in a single-effect of the standard or horizontal type, with a capacity of about H gal. per square foot.
Maltose syrup is made from corn flour and malt, and 1 ton of flour with 0.2 tons of distillers malt will give about 8 tons of 10°Balling extract. After proper filtration, this liquor is then concentrated to 50° in a double or triple effect of the horizontal or semi-film type, with a capacity of from 2% to 3J^ gal. per square foot. The 50° extract is concentrated to 80° in a single effect of the vertical or horizontal type.
All the extracts will foam considerably, and special precautions should be taken to prevent serious losses. The semi-film type has been used very successfully. Un-filtered extracts will produce considerable scale on the tubes, consisting mostly of albuminous matter, and this scale has to be removed frequently by either boiling with